The tax plan proposed by Rep. Paul Ryan (Wis.), the newly minted GOP vice presidential candidate, would have slashed Mitt Romney's effective tax rate to about 1 percent in 2010, based on Romney's tax return that year, according to a Roll Call analysis.

The Ryan tax cut, which would shave about 90 percent off of Romney's tax bill, would result from the Wisconsin Republican's "Roadmap for America's Future" proposal to eliminate taxes on capital gains, dividends and interest. Since about 95 percent of Romney's $21.6 million income came from those sources in 2010, he would pay no taxes on the vast majority of his earnings. It's not certain exactly how low Romney's tax bill would go, but his income from other sources amounts to about $1 million, and Ryan's plan would set a new top rate of 25 percent. Romney's total tax bill would have dropped from the $3 million that he paid to a few hundred thousand dollars if Ryan's plan had been in effect.

Ryan also proposes eliminating the estate tax, which would benefit Romney's heirs by tens of millions of dollars.

But President Barack Obama's campaign has already blasted Romney's own more modest tax plans, citing an analysis by the Tax Policy Center that his plan would cut the tax burden on the wealthy at the expense of the middle class. And Obama has made the issue of tax cuts for the wealthy - and the relatively low tax rate already paid by Romney personally - a central focus of his campaign.